The reactions of benzeneselenenyl chloride with three derivatives of tricyclo [4.2. 2.02, 5] deca-3, 7-diene have been investigated in four solvent systems: methylene chloride, acetic acid, acetic acid/LiClO4, and methanol. Under conditions of kinetic control only products of exo-anti attack upon the cyclobutene moiety are isolable when the solvent system is methylene chloride or acetic acid. This observation also holds in acetic acid/LiClO4 except ...